Process of sweetening a sour hydrocarbon distillate



Oct- 27, 1936- D. l.. YABROFF E'r A1.

PROCESS OF SWEETENING A SOUR HYDROCARBON DISTILLATE Filed May 18, 1936 fly ` and in particular deals with the removal of Patented ccs 27, 1936 UNITED STATES PAT NT PROCESS 0F`SWEETENING A SOUR HYDROCABBON DISTILLATE Application May 18, 1936, Serial N0. 80,374

11 Claims.

This invention relates to the removal of weakly acid reacting organic substances from solutions in organic Vliquids, ofthe type of hydrocarbons,

mercaptans from petroleum distillates.

vIt is frequently necessary to eliminate sma quantities of organic acidlclcomponents such as mercaptans, phenols, naphthenic acids, fatty acids, etc., from their solutions in substantiallx neutral hydrocarbon type liquids. The term, substantially neutral hydrocarbon type liquids, es herein used, refers to hydrophobe normally liqiif :i organic substances which are neutral or slightly I a basic, such 4as' the liquid hydrocarbons derived -irom petroleum, benzene, toluene, xylene, substituted normally liquid hydrocarbons which are substantially .insoluble in watenior instance. chlorinated hydrocarbons, of which chlor ethane, ethylene dichloride, trl-chlorethylene, carbon tetral chloride,- chlor propane, chlorbutylene, chlorbenzene, brom benzene, are examples; or nitro hydrocarbons, for example, nitroethane, unitrobenzene; or other nitrogen containing hydrocarbons such as the amylamlnes, aniline. pyridine, petroleum bases, etc.

Heretofore it has been the practice to treat solutions containing .organic `acidic impurities with aqueous or alcoholic alkali hydroxides of various strengths. In many instances, however, alkali hydroxides fall to eliminate the acidic substances. to the desired degree. 'I'he reason commonly' assigned to this shortcoming was that the acidities of some of the acids are too low to enable their combining with the alkali hygroxides.

An improvement was brought about by' employing countercurrent extraction and by using large excesses of hydroxide solutions over the amounts of liquid to be extracted. However, this treatment is cumbersome and expensive because it requires the handling of excessively large quantities of chemicals, and in some instances such as in the sweetening of sour West Texas gasoline, which involves extractions oi mercaptans having 5 or more carbon atoms, it was found practically impossible to eiect sweetening by caustic treat- 1 ment only, even with a large excess of aqueous caustic. j

Wehave discovered that the true reason for the diillculty of extracting certain of the organic acids from their solutions is not one of excessively low acidity, but of insolubility'in the extraction solvent,'this insolubility being responsible for an extraction solvent and the original solution. We y 55 unfavorable distribution of the acid betwee the have overcome thisL unfavorable @trlblltivn by such, or in suitable solution and, if desired, in

combination with alkali hydroxdes or other strong bases, such as ammonia, calcium hydroxide, etc. 'I'he extraction may be carried out at l0 any temperature below the boiling points of the liquids under the conditions of the treatment. We prefer, however, to extract at temperatures not substantially higher than normal room temperatures, extraction efliciencies declining with l5 increasing temperatures. Since practically all quaternary ammonium bases are solids at room temperature, we normally prefer to use them in solution of a suitable solvent, particularly water, although other non-acid. solvents such as anhydrous liquid .ammonia or aqueous organic solvents which are miscible with water in all proportions and substantially non-miscible with hydrocarbon type liquids, may also be used, of which organic liquids the following are examples: lower alcohols, comprising the monoand poly-hydric alcohols which are liquid at normal room temperatures, specically, methyl, ethyl, propyl alcohols, glycol, glycerine; chlorhydrln; amino alcohols, alcohol ethers, such as ethylene, glycol mono-ethyl ether, lower keton'es such as acetone, methyl ethyl ketone; lower di-amines, such as ethylene diamine, etc.

Some of the hydrocarbon type liquids and organic solvents for the base as herein mentioned. may be miscible with each other. Since, however, the miscibility properties iof these liquids are generally well known, it shall be within the skill of the operator to choose from the given list those solvents which are substantially non- 40 miscible with a particular type hydrocarbon liquid to be extracted.

'I'he peculiar suitability of quaternary ammonium bases for the purpose of extracting organic acids, as herein described, appears to be due to a combination of three properties, namely, solubility in water, high alkalinity and selective solveht power for organic compounds containing polar substitution groups. The relative iniuence of these properties is discussed below.

Solubility in water is important, because water besides being a cheap solvent, renders the quaternary ammonium base fully immiscible with hydro-carbon type liquidsthereby preventing losses of the former.

The extraction of an acidic solute RH from the solution in a hydrocarbon type liquid by means of an aqueous base solution, can be formulated by the following equilibria:

(l) (2) RH RH RB hydrocarbon aqueous phase phase This formula,l in which R is an organic acid radical and B is a base radical indicates that there are at least two main equilibria, marked (1) and (2) respectively, which control the extraction efliciency. Equilibrium (l) represents the distribution of unneutralized free acidlbetween the hydrocarbon and aqueous phases. It depends upon the relative solubilities of the free ,Y acid in the two phases. Since vthe solubility of `insoluble in theI hydrocarbon liquid, high alkalinity of the base is therefore important. Assuming that equilibrium (2) is constant, it will be seen that the i'lnal equilibrium between solute RH in the hydrocarbon phase, and solutes RH and RB in the aqueous phase, is then solely dependent on the solubility of the free acid in the aqueousphase. In other words, for a given degree of hydrolysis of the salt RB in the aqueous phase, the partition coecient which is concentration of solute in the aqueous phase per concentration of the solute in the hydrocarbon phase depends only on the solvent power of the aqueous phase for the unneutralized free acid.

Quaternary ammonium bases suitable for the described purpose, have the formula in which R1 to R4 are alkyl, unsaturated alkyl, aryl, or aralkyl radicals, which may contain polar substitution groups selected from' the class of -OI-I, -NH2, NO2 and halogen, or heterocyclic radicals which are linked to the quaternary nitrogen atom by way of a carbon atom, which carbon atom may or may not be part of the heterocyclic ring. The lower ones of these basesI are soluble.

in water, solubility tending to decrease with increasing 'size of the organic radical.

We have found that the nature of the radical has a considerable influence on the solvent power of lthe quaternary ammoniumbases for organic compounds, such as the organic acids of the type groups, at least one of which radicals is aromatic.

I For instance. we prefer trimethyl tolyl ammonium hydroxide to either tetramethyl or trimethyl cresyl ammonium hydroxide, although even the latter give excellent results.

The concentration of quaternary ammonium base in an aqueous solvent required to elfect substantially complete extraction of an organic acid i solute from an organic liquid depends on the nature of the solute. Obviously the solvent effeet increases with increasing concentration. In some instances a aqueous solution may prove sufficient, but normally we prefer to employ solutions of concentrations of the order of 30 to 50%. Since, however, there is no general Arule by which to define an optimum concentration, economical factors having to be considered, We do not wish to be limited by any specific concentration range.

As previously Ypointed out, high alkalinity of the base solution is desirable. To increase the alkalinity, we often add to the base solution an alkali hydroxide, which addition greatly enhances the extraction efficiency.

In order to explain the importance of our invention more fully, we resort to the theory of countercurrent extraction of two mutually immiscible liquids, namely, a feed containing the solute to be extracted'and the extraction solvent which flow in countercurrent to Y, each other through a multi-stage extraction apparatus. The mathematical treatment of this problem leads to the formula:

% solute left in feed= 1 SK 1 (sm-n 1 in which S=solvent to feed ratio K=partition coecient= concentration of solute in extraction phase concentration of solute in raffinate phase n=number of stages In the attached drawing, to which it is now referred, a graph is showni in which the product SK of solvent to feed ratio S and partition coeflicient K is plotted against percent solute removed frorn the feed for the various numbers of extraction stages n, in accordance with the above formula. From this drawing it will be seen that the percent removed increases rapidly with increasing SK. Thus, SK should be as high as possible, at least higher than 1, because otherwise all of the solute cannot be removed from the feed even with infinite number of stages. Since, however, S should be as low as possible to minimize size of treating equipment, pumping cost and loss of solvent, it is necessary that vK be high. Moreover, the number of extraction stages should be reduced to a minimum to reduce the cost of equipment, which demands a further increase in the value of K.

Applying the graph to the extraction of mercaptans and knowing that the maximum amount of mercaptan sulfur tolerable in a sweet hydrocarbon distillate is .0004%,' it is possible to determine the solvent to' feedy ratio and number of stages which are necessary to `produce a "doctor sweet distillate, if the average K value of the solvent against the hydrocarbon distillate for the mercaptans therein contained is known.

The following examples, in which the advantage of .extracting mercaptans with a quaternary ammonium base over extraction with alkali hydroxide is shown, serve to illustrate this point:

It is desired to sweeten a West Texas straight rungasoline containing .07.32% mercaptan sulfur. To eiect sweetening and to reduce the mercaptanv sulfur content to .0004%, 99.45% of the mercaptan sulfur must be removed. According to the graph, a 4stage extraction at an SK value of 4, or a 3-stage extraction at an SK value of 8, gives the desired result.

The average K value for the mercaptans in this above West Texas gasoline is about 9. Again Aas 9' tri-methyl benzyl ammonium hydroxide Avere e in 2.5 normal aqueous sodium hydroxide K w With a K value of 50 the amounts of solvent necessary to sweeten the gasoline are .08 volumes in a 4-stage extraction and .16 volumes in a 3-stage extraction per volume of gasoline, which volumes are about 1/200 of those required to sweeten with 2.5 normal aqueous alkali hydroxide in the absence of quaternary ammonium base,

and about .V6 of those required of the 2.5 normal Quaternary ammonium base in the absence of alkali hydroxide. A 'f Recovery of the quaternary ammonium base can be had by several/methods, which methods depend largely on the\type of acids extracted. If mercaptans are theonly acids contained in the extract, steaming at elevated temperature will remove the largest part. Preferable, however, is

anV oxidation treatment such as electrolytio oxidation; or blowing with air or oxygen, if desired, in the presence of catalysts comprising heavy metals having atleast two oxidation stages, their oxides and salts, particularly copper. During this treatmentmercaptans are converted to disuldes, which disuldes can be removed by washing the base solvent with naphtha and the like or by skimming and/or steaming.

If acids other than mercaptans such as hydrogen suliide, aromatic hydroxy compounds, naphthenic acids, fatty acids,etc. are present. recovery of the quaternaxy ammonium base is more complicated and may require neutralization with mineral acids, vacuum distillation and extraction with hot methyl alcohol.

Since hydrogen sulde and most hydroxy and carboxyl acids are absorbed substantially 'completely from hydrocarbon type liquids by treatment with aqueous alkali hydroxide alone, and since these acids make recovery of the quaternary ammonium base relatively complicated, we often prefer to give the liquid to be treated a wash with an aqueous alkali reacting' substance, such as tri-potassium phosphate, and/or alkali hydroxide, whereby hydrogen sulfide and oxy acids are removed. The liquid so pretreated is then subjected to the extraction with the quaternary ammonium base. whereby sweetening is effected. 'I'he spent quaternary ammonium base containing mercaptans only is then easily regenerated as aforementioned, and substantially pure mercaptans or disulfides can be recovered. Disuldes can be reconverted to mercaptans by known methods, if desired.

We claim as our inventionz,

1. In the process of separating acid reacting organic substances contained in a hydrocarbon ,type liquid by extraction with an alkaline reacting substance, the improvement comprising subjecting said liquid to an extraction with a quateruary ammonium base under conditions to form an extract phase containing a substantial amount of the acid substances, and a raillnate phase, and

` dissolved in a non-acid solvent which is miscible with water in all proportions and is substantially non-miscible withthe hydrocarbon type liquid, said solvent containing a strong inorganic base.

5. The process of claim 1, in which the quaternary ammonium base is dissolved in a non-acid solvent which is miscible with water in all proportions, and is substantially non-miscible with the hydrocarbon type liquid, said solvent containing an alkali hydroxide. L

6. The process oi claim 1, in which the base is dissolvedI in an aqueous alkali hydroxide. '7. In the process of sweetening a sour hydrocarbon distille' containing mercaptans in excess of .0004% mercaptan sulfur by extraction with ran alkaline reacting substance to remove mercaptans, the improvement comprising extracting said distillate with a suiilcient amount of a quaternary ammonium base under conditions to produce an extract phase containing mercaptans, and a rafiinate phase containing not more than .0004% mercaptan sulfur, and separating said phases. i

8. In the process of separating mercaptarisv from a hydrocarbon type liquid containing mercaptans and other acid substances, the improvement comprising treating said liquid with an aqueous alkaline reacting substance capable of absorbing said acid substances, whereby the latter are substantially removed from said liquid containing mercaptans, extracting the resulting treated liquid with a quaternary ammonium base under conditions to produce an extract phase containing mercaptans, and a railinate phase, separating said phases and treating said extract phase to remove mercaptans.

9. In the process of separating mercaptans from a hydrocarbon type liquid containing mercaptans and other acid substances, the improvement comprising treating said liquid' with an aqueous alkaline reacting substance capable of absorbing said acid substances, wherebythe "latter are substantially removed from said liquid containing mercaptans, extracting the resulting treated liquid with a quaternary ammonium base under conditions to produce an extract phase containing mercaptansl and a raiilnate phase, separating said phases, subjecting said extract `phase to an oxidizing treatment to convert mercaptans to disuldes and separating the disuldes from the base.

10. In the process of separating acid reacting organic substances contained in a hydrocarbon type liquid by extraction with an alkaline reactradicalvunder conditions to form an extract phase containing a substantial amount of the acid sub'J stances and a raiiinate phase, and separating said phases.

11. In the process of separating acid reacting organic substances contained in a hydrocarbon type liquid by extraction with an alkaline reacting substance, the improvement comprising subjecting said liquid to an extraction with trimethyl benzyl ammonium hydroxide under conditions to'form an extract phase containing a substantial amount of the acid substances and a railnate phase, and separating said phases.

DAVID LOUIS YABROFF. JOHN WILmSON GIV'ENS. 

